1. Field of the Invention
The present invention relates to an image forming apparatus such as a multifunction copier or a printer and, more particularly, to the paper-feed control of an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses typically have countermeasures to prevent a paper jam. Particularly, when a sheet of paper is fed out from a paper feeder for paper feeding, the sheet of paper can accidentally slip from a carrier path because sufficient carrying power is not transmitted to the sheet of paper, unlike a case where the sheet of paper is held by a carrier roller. In order to overcome the above drawback, a control method is adopted in which the paper feeding is started as soon as possible and the sheet of paper is carried as fast as possible. With this method, even when the fed sheet of paper slips from the carrier path and is delayed, the sheet of paper can arrive at a registration unit within a predetermined time period.
The registration unit suspends the sheet of paper in a state where the sheet of paper impinges against a pair of registration rollers that are stopped, and then restarts rotation of the registration rollers in synchronism with imaging on a photoconductive drum (registration-on) to carry the sheet of paper to the photoconductor drum, in order to align the sheet of paper with an image formed on the photoconductive drum.
If the sheet of paper arrives at the registration unit too early, a subsequent sheet of paper strikes the trailing end of a preceding sheet of paper that is stopped in the registration unit. As a countermeasure against this, a method is known in which the sheet of paper is fed at a speed higher than a processing speed that is used in imaging and transfer, and the subsequent sheet of paper is carried to the registration unit only after the subsequent sheet of paper is stopped at an upstream position of the registration unit before the leading end of the subsequent sheet of paper catches and strikes the trailing end of the preceding sheet of paper stopped at the registration unit. Feeding the sheet of paper at a speed higher than the processing speed is referred to as increased-speed paper feeding. Suspending the subsequent sheet of paper upstream of the registration unit is referred to as pre-registration stop (for example, Japanese Patent Application Publication No. 2002-29649).
Although FIGS. 3 and 4 are diagrams illustrating an embodiment of the present invention, the pre-registration stop will be described below by incorporating FIGS. 3 and 4 by reference.
FIG. 3 illustrates an example of the increased-speed paper feeding and the pre-registration stop. A vertical axis 3001 represents a position of a sheet of paper with respect to the registration rollers. A horizontal axis 3002 represents time. Reference numeral 3003 denotes a registration position. Reference numeral 3004 denotes a pre-registration stop position. Reference numeral 3005 denotes the position of a pre-registration sensor. Stopping of the sheet of paper and creating a loop at the registration position are performed with respect to the position 3005. Reference numeral 3006 denotes a position of a vertical-path lower sensor. Stopping of the sheet of paper at the pre-registration stop position is performed with respect to the position 3006. Reference numeral 3007 denotes the paper-feed starting time of a preceding sheet of paper. Reference numeral 3008 denotes the paper-feed starting time of a subsequent sheet of paper. Reference numeral 3009 denotes a segment showing how the leading end of the preceding sheet of paper proceeds and reference numeral 3010 denotes a segment showing how the leading end of the subsequent sheet of paper proceeds. Reference numeral 3011 denotes a segment showing how the trailing end of the preceding sheet of paper proceeds and reference numeral 3012 denotes a segment showing how the trailing end of the subsequent sheet of paper proceeds. The variation in the inclination before and after the registration position 3003 shows that the speed of the sheet of paper before the registration is different from the speed thereof after the registration. The speed after the registration is the processing speed and the speed before the registration is an increased-paper-feed speed.
Reference numeral 3015 denotes a time period during which registration-on is performed. The distance corresponding to the time period 3015 corresponds to an interval at which the sheet of paper is fed to the registration unit. Reference numeral 3016 denotes a pre-registration-stop time period. When the sheet of paper is delayed due to slip during paper feeding, shortening the pre-registration-stop time period 3016 can eliminate the delay during paper feeding, thus minimizing the delay of the sheet of paper at the registration unit.
However, applying the known increased-speed paper feeding and pre-registration stop technology to recent printers that feed sheets of paper at short intervals in order to achieve a high productivity has caused the following problems.
When the paper-feed starting position is sufficiently apart from the registration position as shown in FIG. 3, no problem is caused if the sheet of paper is fed at shorter intervals. However, there are cases in which the paper-feed starting position is near the registration position. For example, in an image forming apparatus provided with a plurality of paper cassettes at the lower part of a printer, the distance between the paper-feed starting position and the registration position varies from cassette to cassette. The paper-feed starting position of a top paper cassette is nearest to the registration position. The paper-feed starting position that is not sufficiently apart from the registration position causes the following problems.
The same parts as in FIG. 3 are not described in the description with reference to FIG. 4 and the reference numerals in FIG. 4 shall have the same meanings as described above in FIG. 3. Reference numeral 4001 denotes a vertical-path upper sensor, which is a reference sensor for the pre-registration stop with respect to the paper-feed starting position that is nearer to the registration position than the paper-feed starting position in FIG. 3. Reference numeral 4002 denotes the paper-feed starting time of a preceding sheet of paper. Reference numeral 4003 denotes the paper-feed starting time of a subsequent sheet of paper. The time interval between 4002 and 4003 is the same as that between 3007 and 3008 in FIG. 3 and, therefore, the paper feeder in FIG. 3 has the same productivity as the paper feeder in FIG. 4. Referring to FIG. 4, the trailing end of the preceding sheet of paper intersects the leading end of the subsequent sheet of paper at a position 4005. In other words, since the paper-feed starting position is near the registration position, the leading end of the subsequent sheet of paper catches the trailing end of the preceding sheet of paper carried at the processing speed because the subsequent sheet of paper is fed at the increased-paper-feed speed before reaching the pre-registration stop. When the paper-feed starting time is delayed in order to solve the above problem, the productivity cannot be maintained unless a pre-registration-stop time period 4004 is shortened. Shortening the pre-registration-stop time period is likely to cause a paper jam due to the delay. Furthermore, a pre-registration-stop time period that is too short can cause a carrier motor in the pre-registration unit to be out of synchronization with control pulses, thus disadvantageously causing paper jam.